I enabled CA1012 with code existing that broke the rule. I saw a green squigglie as expected.

I then changed this to be an error, and saved the ruleset.

The break correctly appeared in the Error pane as an error and as an error in the Solution Explorer list of active analyzers.

However, the squiggly did not change from green to red until I closed and reopened the file.

It would be nice if this "just worked," but at the least, I think there should be mitigation messages somewhere.